There are known a number of photosensitive elements having a photosensitive layer on a support wherein images are formed by imagewise exposure. Among these, a technique of forming images through heat development is known as a system capable of simplifying image forming means and contributing to the environmental protection.
From the contemporary standpoints of environmental protection and space saving, it is strongly desired in the photomechanical process field to reduce the quantity of spent solution. Needed in this regard is a technology relating to photothermographic elements for use in photomechanical process which can be effectively exposed by means of laser scanners or laser image setters and produce distinct black images having a high resolution and sharpness. These photothermographic elements offer to the customer a simple thermographic system that eliminates a need for solution type chemical agents and is not detrimental to the environment.
The technology of forming images through heat development is disclosed, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, D. Morgan and B. Shely, "Thermally Processed Silver Systems" in "Imaging Processes and Materials," Neblette, 8th Ed., Sturge, V. Walworth and A. Shepp Ed., page 2, 1969. These photothermographic elements generally contain a reducible non-photosensitive silver source (e.g., organic silver salt), a catalytic amount of a photocatalyst (e.g., silver halide), and a reducing agent for silver, typically dispersed in an organic binder matrix. Photothermographic elements are stable at room temperature. When they are heated at an elevated temperature (e.g., 80.degree. C. or higher) after exposure, redox reaction takes place between the reducible silver source (functioning as an oxidizing agent) and the reducing agent to form silver. This redox reaction is promoted by the catalysis of a latent image produced by exposure. Silver formed by reaction of the reducible silver salt in exposed regions provides black images in contrast to unexposed regions, forming an image.
Photothermographic elements of this type, however, have soft photographic characteristics and are not acceptable in the graphic art application requiring high contrast.
As one solution to this problem, a thermographic technique of using nucleating agents to provide ultrahigh contrast characteristics is disclosed in U.S. Pat. Nos. 5,496,695, 5,536,622, 5,545,515, and 5,635,339. In most of these elements, photosensitive layers are formed by applying coating solutions based on organic solvents such as toluene, methyl ethyl ketone (MEK) and methanol, followed by drying. The use of organic solvents is not only harmful to workers in the manufacturing procedure, but disadvantageous because of the cost for recovery and disposal of the solvents.
It is contemplated to form photosensitive layers using coating solutions based on water solvent which eliminates such concern. Such photosensitive layers are sometimes referred to as "aqueous photosensitive layers," hereinafter. For example, JP-A 52626/1974 and 116144/1978 disclose the use of gelatin as the binder. JP-A 151138/1975 discloses polyvinyl alcohol as the binder. Further, JP-A 61747/1985 discloses a combined use of gelatin and polyvinyl alcohol. Besides, JP-A 28737/1983 discloses a photosensitive layer containing water-soluble polyvinyl acetal as the binder. It is true that the use of these binders has great environmental and economical advantages in that photosensitive layers can be formed using aqueous coating solutions.
However, the use of such polymers as gelatin, polyvinyl alcohol and water-soluble polyacetal as the binder results in photosensitive materials which are of extremely low commodity worth in that a coating whose surface quality is practically acceptable is not available since these polymers are less compatible with the organic silver salt, and that exposed areas have a low blackened density and unexposed areas have a high density.
The photothermographic elements having ultrahigh contrast characteristics disclosed in the above patents show good photographic performance under optimum heat development conditions. However, they suffer from abrupt changes of percent dot and abrupt increases of Dmin with slight changes of the developing time and temperature.
There is a desire to develop a photothermographic element or aqueous photosensitive element having environmental and economical benefits and good coating surface quality, satisfactory photographic properties including an ultrahigh contrast and low fog, and a wide latitude against variations of development conditions.